RELATIONSHIPS BETWEEN NITRATE REDUCTASE-ACTIVITY AND RATES OF GROWTH AND NITRATE INCORPORATION UNDER STEADY-STATE LIGHT OR NITRATE LIMITATION IN THE MARINE DIATOM THALASSIOSIRA-PSEUDONANA (BACILLARIOPHYCEAE)
RELATIONSHIPS BETWEEN NITRATE REDUCTASE-ACTIVITY AND RATES OF GROWTH AND NITRATE INCORPORATION UNDER STEADY-STATE LIGHT OR NITRATE LIMITATION IN THE MARINE DIATOM THALASSIOSIRA-PSEUDONANA (BACILLARIOPHYCEAE)
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DOI：
10.1111/j.0022-3646.1995.00085.x
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发表时间：
1995-02-01
影响因子：
2.9
通讯作者：
HARRISON, PJ
中科院分区：
文献类型：
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作者：
BERGES, JA;HARRISON, PJ
虽然硝酸还原酶(NR)的活性可以用来预测海洋浮游植物野外组合中硝酸盐的掺入速率，但由于在稳态条件下这两个速率之间的关系还没有很好地建立起来，所以这一指标的应用并不成功。为了给硝酸还原酶活力的测定提供依据，研究了假海链藻和海姆达尔的硝酸还原酶活性、生长速率、细胞组成和硝酸盐掺入率之间的关系，其中a)在稳态光照限制下生长，b)在低光照和高光照(15或90 mmolquant(-2)m(-2)S(-1))之间转换，c)在稳态硝酸盐限制下生长。采用添加牛血清白蛋白来稳定酶活性的NR改良方法，光限制培养中的NR活性与计算的硝酸盐掺入速率呈正相关和定量相关，即使在明显缺硒的培养物中也是如此。在光照转换期间，生长速率在1d内适应了新的条件，硝酸根活性与硝酸盐掺入率之间的关系仍然是定量的。在硝酸盐限制的恒化器培养中，硝酸还原酶活性与生长速率和硝酸盐掺入速率呈正相关，但关系不是定量的。在较低的生长速率(营养充足生长速率的25%)下，硝态氮活性超过硝酸盐掺入速率，但在如此低的稀释率下运行的化学稳定剂可能不代表海洋硅藻的生态相关条件。硝酸还原酶活性与硝酸盐掺入之间的强烈关系支持了硝酸根还原是硝酸盐掺入的速率限制或与速率限制步骤紧密耦合的观点。为了确定一个合适的变量来衡量NR活性，研究了不同细胞成分和生长速度之间的关系。这些关系因限制因素的不同而不同。例如，在光限制下，细胞体积和细胞碳含量随着生长速率的增加而显著增加，而在硝酸盐限制下，细胞体积和碳含量随着生长速度的增加而减少。尽管在光照和硝酸盐限制下，细胞组成和生长速率存在差异，但不同组成变量的NR活性与生长速率之间的关系在限制条件下没有差异。在不容易确定细胞数量的田间情况下，将NR活性调整为颗粒氮含量可能是最好的选择。这些结果为将硝态氮活性测定作为田间硝态氮掺入指标奠定了坚实的基础。
Although activity of the enzyme nitrate reductase (NR) can potentially be used to predict the rate of nitrate incorporation in field assemblages of marine phytoplankton, application of this index has met with little success because the relationship between the two rates is not well established under steady-state conditions. To provide a basis for using NR activity measurements, the relationships among NR activity, growth rate, cell composition, and nitrate incorporation rate were examined in cultures of Thalassiosira pseudonana (Hustedt) Hasle and Heimdal, growing a) under steady-state light limitation, b) during transitions between low and high irradiance (15 or 90 mu mol quant(.)m(-2.)s(-1)), and c) under steady-state nitrate limitation. Using a modified assay for NR involving additions of bovine serum albumin to stabilize enzyme activity, NR activity in light-limited cultures was positively and quantitatively related to calculated rates of nitrate incorporation, even in cultures that were apparently, starved of selenium. During transitions in irradiance, growth rates acclimated to new conditions within 1 day through the transition, the relationship between NR activity and nitrate incorporation rate remained quantitative. In nitrate-limited chemostat cultures, NR activity was positively correlated with growth rate and with nitrate incorporation rates, but the relationship was not quantitative. NR activity exceeded nitrate incorporation rates at lower growth rates (< 25% of nutrient-replete growth rates), but chemostats operating at such low dilution rates may not represent ecologically relevant conditions for marine diatoms. The strong relationship between NR activity and nitrate incorporation provides support for the idea that NR is rate-limiting for nitrate incorporation or is closely coupled to the rate-limiting step. In an effort to determine a suitable variable for scaling NR activity, relationships between different cell components and growth rate were examined. These relationships differed depending on the limiting factor. For example, under light limitation, cell volume and cell carbon content increased significantly with increased growth rate, while under nitrate limitation cell volume and carbon content decreased as growth rates increased. Despite the differences found between cell composition and growth rate under light and nitrate limitation, the relationships between NR activity scaled to different compositional variables and growth rate did not differ between the limitations. In field situations where cell numbers are not easily determined, scaling NR activity to particulate nitrogen content may be the best alternative. These results establish a strong basis for pursuing NR activity measurements as indices of nitrate incorporation in field.